Dear Sir,
I am Egyptian and live in USA, There are some people who converted to Islam elhamdulelah, and we were discussing their marriage. They have 4-5 kids and their husbands are Christians.
How can I tell them that this haram?

Honorable sister,
It is prohibited for the Muslim wife to have physical contact (marital relationship) with her non-Muslim husband as per the words of Allah Almighty, "they are not lawful [wives] for them, nor are they lawful [husbands] for them." (Qur'an 60:10).
A Muslim wife is not to seek a divorce except until she informs the judge about her Islam and thus the judge will inform the husband. The judge is to invite the husband to Islam, or she can invite her husband to Islam by herself. It is desirable that the wife does this through kind word and wisdom. If he accepts it, their marital life is to be continued based on the previously conducted marriage contract; otherwise, the judge is to issue a ruling to separate them. This separation is an irrevocable divorce and the woman's waiting period ('Idda) starts on the same day the judge gives her divorce.
On the other hand, a husband who reverts to Islam is not to divorce his Christian wife if she did not accept Islam.
It is permissible in Islam for a Muslim man to marry a non-Muslim woman (Christian or Jewish) and not vice-versa. Though this may seem unfair, the rationale behind it becomes clear if the true reason is known.
All legislations in Islam are based upon certain wisdom and a definite interest to all parties involved. Marriage in Islam is based upon love, mercy and peace of mind; a family must be built upon a firm basis to guarantee the continuity of the marital relationship. Islam respects all the previous heavenly revealed religions and the belief in all the previous Prophets is an inseparable part of the Islamic creed.
A Muslim man who marries a Christian or a Jewish woman, is commanded to respect her faith, and it is not permissible for him to prevent his wife from practicing the rites of her religion and going to the church or synagogue. Therefore, Islam seeks to provide the wife with her husband's respect for her religion which in turn protects the family from destruction.
On the other hand, a non-Muslim man will not respect his Muslim wife's faith. This is because a Muslim man believes in all previous religions and Prophets of Allah and respects them while a non-Muslim does not believe nor acknowledges the Prophet of Islam; rather, a non-Muslim considers Prophet Mohammed a false prophet and usually believes in all the fabricated lies made against Islam and its Prophet. Even if a non-Muslim husband does not explicitly express this, a Muslim wife will constantly feel that her husband does not respect her faith. There is no room for compliments regarding this matter; it is a matter of principle. Moreover, mutual respect between spouses is a fundamental element for the continuity of their marital relationship.
Islam follows its own logic when it prohibits a Muslim man from marrying a non-Muslim other than a Christian or a Jewess for the same reason it prohibits a Muslim woman from marrying a non-Muslim.
A Muslim believes in only the Heavenly revealed religions; all other religions are human made. So, in the case when a Muslim woman marries a non-Muslim, the element of respect to the wife's religion will be non-existent. This will affect the marital relationship and will not achieve the love and mercy that is required in a marital relationship.
You should choose the best way to inform each one of those sisters. This includes advising them to contact Dar al-Ifta al-Missiriyah via this section without engaging in answering their queries yourself. Dar al-Ifta al-Missiriyah provides answers which are in accord with everyone's culture, mannerism and way of communicating his issue.